1 Field of the Invention
The present invention relates to a sealing agent useful for liquid crystal displays using a plastic film or sheet as a substrate, and to a liquid crystal display using the sealing agent.
2 Discussion of the Background
Liquid crystal displays are widely used as display devices for personal computers, word processors and the like because they are thin and light weight, and consume low power. Among these liquid crystal displays, displays in which glass is used as the substrate of their liquid crystal cells have been typically used (hereinafter referred to as glass type LCDs). Currently, as so-called xe2x80x9cmobile devicesxe2x80x9d are developed and liquid crystal displays are used in the mobile devices, liquid crystal display cells in which a plastic film or sheet is used as the substrate (hereinafter referred to as film type LCDs) have been increasingly used because of their light weight, good flexibility, and resistance to being easily crushed.
Liquid crystal displays are used while liquid crystal is sealed with a sealing agent in a substrate of the liquid crystal displays. The sealing agent preferably has good adhesive strength so as not to be destroyed by a stress applied to the liquid crystal displays using the sealing agent. In addition, the sealing agent is required not to adversely affect molecules of the liquid crystal contacting the sealing agent. Further, the sealing agent which is used for liquid crystal displays having a plastic film substrate preferably has flexibility because the displays are used while they are bent. Furthermore, the sealing agent which is used for liquid crystal displays useful for mobile devices is required to have maximum heat resistance. Thermosetting epoxy resins have been typically used as a sealing agent for liquid crystal displays. In film type LCDs, the crosslinking temperature of the sealing agent cannot be set to be as high as for glass type LCDs because the film or sheet substrate has relatively poor heat resistance compared to the glass substrate. Therefore the sealing agent for the film type LCDs has a drawback of poor heat resistance.
In addition, film type LCDs are required to have flexibility, which glass type LCDs are not required to have. Therefore sealing agents which are different in kind from those used for glass type LCDs have been used for film type LCDs, or sealing agents which are the same kind as those used for glass type LCDs have been used while being crosslinked at a relatively low temperature compared to that for the glass type LCDs. When such film type LCDs are preserved at a high temperature, which is a requisite for the sealing agent, liquid crystal tends to impregnate into the sealing agent, and thereby the sealing agent swells, resulting in decrease of the adhesive strength of the sealing agent layer (a problem hereinafter termed xe2x80x9cpoor resistance to liquid crystalsxe2x80x9d). In addition, in this case, the switching current of the liquid crystal tends to increase because of the mixing of the sealing agent in the liquid crystal, resulting in malfunction of the film type LCDs (a problem hereinafter termed xe2x80x9ca liquid crystal contamination problemxe2x80x9d). Therefore a need exists for a sealing agent having good reliability, without causing these problems.
In attempting to solve these problems, Japanese Laid-Open Patent Publication No. 62-18523 discloses polyol type epoxy resins serving as a sealing agent for film type LCDs. The sealing agent has good flexibility and adhesive strength; however, the sealing agent swells and the switching current of the liquid crystal gradually increases when the film type LCD using the sealing agent is preserved at about 80xc2x0 C. Thus, this sealing agent cannot entirely address the problem of poor heat resistance.
Japanese Laid-Open Patent Application No. 9-12679 discloses a sealing agent for film type LCDs which includes as the essential components 10 to 50 parts by weight of a silicone-modified epoxy resin which is liquid at room temperature, 90 to 50 parts by weight of a bisphenol type epoxy resin which is also liquid at room temperature, 20 to 80 parts by weight of a trifunctional thiol type crosslinking agent which is also liquid at room temperature, 0.5 to 5.0 parts by weight of a silane coupling agent, 1 to 10 parts by weight of amorphous silica having an average particle diameter not greater than 1 xcexcm, and 5 to 50 parts by weight of an inorganic filler other than amorphous silica, which has an average particle diameter not greater than 2 xcexcm. However, the sealing agent also does not entirely improve the heat resistance because the switching current of the liquid crystal gradually increases when the film type LCD using the sealing agent is preserved at about 80xc2x0 C.
Because of these reasons, a need exists for a sealing agent for film type LCDs which has good flexibility and good heat resistance as well as good resistance to liquid crystals. In addition, a need exists for a film type LCD which successfully addresses the problems mentioned above.
Accordingly, an object of the present invention is to provide a sealing agent useful for film type LCDs which has good flexibility and good heat resistance as well as good resistance to liquid crystals even when the cells of the film type LCDs are manufactured by an assembling method including heat treatment at a conventional heating temperature.
Another object of the present invention is to provide a film type LCD which uses the above-mentioned sealing agent and which has good heat resistance and good resistance to liquid crystals, and successfully addresses a liquid crystal contamination problem.
Briefly these objects and other objects of the present invention as hereinafter will become more readily apparent can be attained by a sealing agent which includes at least epoxy ingredients of a bisphenol type epoxy resin and a non-bisphenol type epoxy resin, and crosslinking agent ingredients of an aromatic amine compound and/or an alicyclic amine compound.
The non-bisphenol type epoxy resin preferably includes an alcohol type epoxy resin, a polysulfide modified epoxy resin or a silicone oil modified epoxy resin.
The sealing agent preferably includes a crosslinking catalyst including an acid.
In another aspect of the present invention, a film type LCD which includes a plastic film substrate in which a liquid crystal is sealed with the sealing agent mentioned above is also provided.
Other features of the invention will become apparent in the course of the following descriptions of preferred embodiments, which are given for the purpose of illustration only and are not intended to be limiting.
The present inventors discovered that the above-described problems can be solved by a sealing agent which includes epoxy ingredients including specified epoxy resins, and a crosslinking agent including a specified amine compound.
In detail description, it was discovered that the resistance to liquid crystals and the liquid crystal contamination problem can be successfully addressed by a sealing agent including a bisphenol type epoxy resin, and an aromatic amine type crosslinking agent and/or an alicyclic amine type crosslinking agent. The reason is considered to be that the bisphenol type epoxy resin has a high cohesive force due to having aromatic rings therein and is inactive because of having a high glass transition temperature; thereby the epoxy resins tend to be hardly impregnated with molecules of liquid crystal, resulting in improvement of the resistance to liquid crystals and the liquid crystal contamination problem. By using an aromatic amine and/or alicyclic amine as a crosslinking agent, this effect can be enhanced.
The combination of the bisphenol type epoxy resin and the crosslinking agents of the aromatic amine and/or the alicyclic amine, however, has a drawback in that the resultant sealing agent becomes too hard after crosslinking and tends to be peeled from a substrate when the substrate is stressed.
It was also discovered that this peeling problem can be solved by adding a non-bisphenol type epoxy resin to the bisphenol type epoxy resin. Thus a sealing agent having good resistance to liquid crystals and good resistance to peeling can be obtained.
Thus, there is no criticality with regard to utility in the relative amounts of bisphenol type epoxy resin and non-bisphenol type epoxy resin, so long as each is present in amounts sufficient to produce the above-described effects. Similarly, there is no criticality with regard to utility in the amount of crosslinking agent, so long as it is present in crosslinking amounts.
The term xe2x80x9cbisphenol type epoxy resinxe2x80x9d herein is intended to have a meaning well-defined in the art, i.e., it contains groups derived from a bisphenol. Similarly, the term xe2x80x9cnon-bisphenol type epoxy resinxe2x80x9d herein is intended to have a meaning well-defined in the art, i.e., it does not contain groups derived from a bisphenol.
Suitable bisphenol type epoxy resins for use in the present invention are not particularly limited and include known bisphenol type epoxy resins. For example, bisphenol A type epoxy resins having the following formula (1) and bisphenol F type epoxy resins having the following formula (2) can be employed.
Bisphenol A Type Epoxy Resin 
wherein n is 0 or 1.
Bisphenol F Type Epoxy Resin 
wherein n is 0 or 1.
Specific examples of such bisphenol A type epoxy resins include EPIKOTE 828, 834, 1001 and 1004, which are manufactured by YUKA SHELL EPOXY KK, and EPICLON 850, 860 and 4055, which are manufactured by Dainippon Ink and Chemicals Inc.
Specific examples of such bisphenol F type epoxy resins include EPIKOTE 807, which is manufactured by YUKA SHELL EPOXY) KK, and EPICLON 830, which is manufactured by Dainippon Ink and Chemicals Inc.
These epoxy resins may be employed alone or in combination.
Suitable non-bisphenol type epoxy resins for use in the present invention include alcohol type epoxy resins, polysulfide modified epoxy resins, silicone oil modified epoxy resins and the like. The term xe2x80x9calcohol type epoxy resinsxe2x80x9d is intended to have a meaning well-defined in the art, i.e., it contains groups derived from an alcoholic hydroxy group.
The alcohol type epoxy resins for use in the present invention include compounds which have a glycidyl ether group which is obtained, for example, by a reaction of an alcoholic hydroxy group and an epichlorohydrin group. Specific examples of such compounds include compounds having a formula selected from the following formulas (3) to (6):
Polyethylene Glycol Type Epoxy Resins 
wherein n is an integer of from 5 to 15.
Propylene Glycol Type Epoxy Resins 
wherein n and m are an integer and the total number of n and m is 3. 
wherein n is 7.
1,6-hexanediol Digycidylether Type Epoxy Resins 
The polysulfide modified epoxy resins for use in the present invention include compounds having a group xe2x80x94Sxe2x80x94 in the skeleton of the molecule thereof and epoxy groups (or glycidyl groups) at both ends of the molecule thereof. Specific examples of such compounds include polysulfide modified epoxy resins having the following formula (7): 
wherein n is an integer of from 1 to 10.
Specific examples of such polysulfide modified epoxy resins include FLEP-10, FLEP-50, FLEP-60 and FLEP-80 manufactured by TORAY THIOKOL Co., Ltd.
The silicone oil modified epoxy resins for use in the present invention include compounds having a group xe2x80x94SiOxe2x80x94 in the skeleton of the molecule thereof and epoxy groups (or glycidyl groups) at both ends of the molecule thereof. Specific examples of such compounds include compounds having the following formula (8): 
wherein R represents a methyl group or a phenyl group; and n is an integer of from 1 to 9.
These bisphenol type and non-bisphenol type epoxy resins may include one or more epoxy resins, respectively.
Suitable aromatic amines for use as a crosslinking agent in the present invention include diaminodiphenylmethane (DDM), metaphenylenediamine, diaminodiphenylsulfone, 3, 3xe2x80x2dimethyl-5,5xe2x80x2-diethyl-4,4xe2x80x2-diaminophenylmethane, dimethylethylmethaphenylenediamine, epoxy-adducts of DDM, and the like.
Suitable alicyclic amines for use as a crosslinking agent in the present invention include isophoronediamine, norbornenediamine, cyclohexylenediamine, dimethyldiaminodicyclohexylmethane and the like.
The sealing agent of the present invention may include a crosslinking catalyst to shorten the crosslinking time of the sealing agent and/or to lower the crosslinking temperature. Specific examples of such a crosslinking catalyst include inorganic solid acids such as silica, and organic acids such as salicylic acid.
By including the acid mentioned above in a sealing agent as a crosslinking catalyst, the sealing agent can be crosslinked at a low temperature, for example, lower than 40xc2x0 C. When a crosslinking catalyst is not employed, sealing agents including an aromatic amine type crosslinking agent such as metaphenylenediamine and diaminodiphenylmethane need to be crosslinked at a temperature higher than 60xc2x0 C. However, when a crosslinking catalyst is employed together with the crosslinking agent, the sealing agent can be crosslinked at 40xc2x0 C.
In this crosslinking (first crosslinking), the LCD cell is baked at about 40xc2x0 C. to obtain a uniform cell gap while the substrate is pressed. Therefore it is preferable to perform the first crosslinking of the sealing agent at such a low temperature as 40xc2x0 C. or lower because problems such as deformation of the substrate and the so-called xe2x80x9cbruisexe2x80x9d of the cell can be avoided.
When a particulate silica is used as a crosslinking catalyst, the silica can serve as an acid catalyst because hydroxide groups present on the surface of the silica dissociate to produce hydrogen ions. The particulate silica can also serve as a thickener which is mentioned below.
The sealing agent of the present invention may include a filler to obtain good printing qualities. The sealing agents of LCD cells are formed on substrates of the cells, typically, by a screen printing method. Therefore the viscosity of the sealing agent to be printed is preferably from about 30 to about 50 Paxc2x7s. The viscosity can be controlled by adding a filler to the sealing agent. Specific examples of such fillers include titanium oxide, silica, alumina, calcium carbonate and the like. These fillers can be used alone or in combination.
The fillers can be added to the epoxy ingredients and/or the crosslinking agent, or the mixture of the epoxy ingredients and the crosslinking agent when a sealing agent printing liquid is prepared. A homogenizer or a 3-roll mill is preferably used for mixing the filler and the epoxy ingredients and/or the crosslinking agent. The content of the filler in the sealing agent printing liquid is preferably from 0 to 100 parts by weight per 100 parts by weight of the epoxy ingredients or the crosslinking agent.
The sealing agent of the present invention may include a silane coupling agent to improve adhesive strength. Specific examples of the silane coupling agent include:
xcex3-glycidoxypropyltrimethoxysilane,
xcex3-glycidoxypropyltriethoxysilane,
xcex3-glycidoxypropylmethyldiethoxysilane,
xcex2(3,4-epoxycyclohexyl)ethyltrimethoxysilane,
xcex3-aminopropyltrimethoxysilane,
N-xcex2-(aminoethyl)-xcex3-aminopropyltrimethoxysilane, and the like.
The content of the silane coupling agent in the sealing agent is generally from 0.5 to 20 parts by weight, and preferably from 1 to 10 parts by weight, per 100 parts by weight of the epoxy ingredients.
Suitable resin films for use as the substrate of the cells of the film type LCDs of the present invention include polycarbonate films, polyethersulfone films, polysulfone films and the like. Since these films have large gas transmittance and poor resistance to solvents, a gas barrier layer and/or a protective layer made by an inorganic or organic material can be formed. In addition, an ITO layer is formed thereon. The total thickness of the substrate is from about 0.1 to about 0.3 mm.
As for a method for measuring the resistance to liquid crystals, the following method has been developed:
(1) crosslinking a sealing agent;
(2) weighing the crosslinked sealing agent;
(3) dipping the crosslinked sealing agent in a liquid crystal at a high temperature for a predetermined time; and
(4) then measuring the resistivity of the liquid crystal, or weighing the sealing agent after the preservation to determine the weight ratio of the sealing agent after the preservation to that before preservation (hereinafter this weight ratio is referred to as xe2x80x9cswelling degreexe2x80x9d).
When the resistance of the sealing agent to liquid crystals is poor, the resistivity of the liquid crystal after the preservation is relatively low and the swelling degree of the sealing agent is relatively large, because one or more ingredients of the sealing agent migrate into the liquid crystal, and the sealing agent is impregnated with the liquid crystal, resulting in swelling of the sealing agent. Namely, the higher the resistivity of the liquid crystal or the more closely the swelling degree approaches 1, the better the resistance of the sealing agent to the liquid crystal.
The resin film type LCD (hereinafter PF-LCD) of the present invention can be manufactured, for example, by the following method.
One or more resin films which are formed with an ITO electrode are used as a substrate of the LCD. A pattern is formed thereon by any known method such as photolithography and the like. After the substrate is washed, an orientating layer which is of any pattern and which is made of polyamide, polyimide or the like is printed thereon by flexagraphy. The substrate is baked at a predetermined temperature and then subjected to rubbing treatment. The dust of the rubbing treatment is ultrasonically dry-cleaned. Windows are formed in the substrate for setting an electrode terminal therein (this process hereinafter referred to as a xe2x80x9cprecutting processxe2x80x9d).
In addition, in order to perfectly remove dust, the substrate is ultrasonically cleaned with water (ultrasonic cleaning with water at frequencies of from 30 to 60 KHZ followed by ultrasonic cleaning with water at frequencies of about 1 MHZ).
Spherical particles of a resin or an inorganic material are sprinkled on one of the film substrates to make the gap of a resultant LCD cell uniform. The sealing agent of the present invention is formed on the other film substrate by a screen printing and then the two substrates are laminated with each other so that the two substrates are aligned. The pair of substrates laminated are set in an air bag type pressure applying oven, and then sealed and baked at a temperature not higher than 60xc2x0 C. The pair of substrates sealed are cut to form a plurality of cells (when LCD cells are manufactured, a method in which a large substrate consisting of a plurality of cells are cut to form LCD cells is generally used). The cut cells are further baked at a high temperature higher than 100xc2x0 C. to accelerate the crosslinking reaction of the sealing agent.) In this case the cells are baked without pressure while being contained in a jig such as a cassette.
A liquid crystal is charged into the cells by a vacuum charging method utilizing pressure difference. After the charging of the liquid crystal, the cell is excessively charged; with the liquid crystal and the gap of the cell is not uniform. Therefore the cell is pressed to extrude the excess liquid crystal therefrom and then sealed. In this case the closing is preferably performed with an ultraviolet crosslinking resin, and may be performed with an epoxy type heat crosslinking resin.
A deflecting plate with or without a phase plate is adhered on the front side of the cells (a deflecting plate is used for TN type liquid crystals and a deflecting plate with a phase plate for STN type liquid crystals). On the rear side of the cells, a deflecting plate with a reflection plate or a semi-transparent plate is adhered to form a reflection type or a semi-transparent panel. The thus formed LCD unit is then implemented with a driving circuit and a connector connecting the driving circuit with the unit.
Currently, various thin information terminals such as portable phones, personal handy phones, card pagers and the like are developed and manufactured. Liquid crystals suitable for these terminals preferably are of STN type whose duty ratio is from 1/16 to 1/64 and which can work with a low voltage and have quick responding properties.
A liquid crystal material which is charged in a cell of an LCD is typically made by mixing about 10 kinds of liquid crystals while controlling the mixing ratio, which depends on a requisite property of the PF-LCD. It is discovered that when the requisite property is low-voltage operating or quick response, the liquid crystal material tends to be affected by materials surrounding the liquid crystal material. In detail description, when the liquid crystal material having a threshold voltage (Vth) not greater than 1.5 V and dielectric anisotropy xcex94∈ not less than 10 is used, the liquid crystal material tends to be particularly affected by the surrounding materials such as a sealing agent, and problems tend to occur in that current consumption of the LCD increases due to the increase of the resistivity of the liquid crystal material and deflecting properties thereof deteriorate. Since the sealing agent of the present invention hardly affects liquid crystal materials, the sealing agent is particularly effective for PF-LCDs, which includes a liquid crystal material which can work at a low voltage and which tends to be affected by the surrounding materials. Namely, by using the sealing agent of the present invention for a PF-LCD, the PF-LCD has good reliability, which cannot be attained by conventional sealing agents.